Search Results (446)

Gas turbines in land-based microgrids and shipboard-isolated power grids frequently face operational challenges, such as the startup and shutdown of high-power equipment and sudden load fluctuations, which significantly impact their performance. To examine the dynamic behavior of gas turbines under transitional operating conditions, a three-dimensional computational fluid dynamic simulation is employed to create a model of the gas turbine rotor, incorporating thermal inertia, which is then analyzed in conjunction with three-dimensional finite element methods. The governing equations of the flow field are discretized, providing results for the flow and temperature fields throughout the entire flow path. A hybrid approach, combining temperature differences and heat flux density, is applied to set the thermal boundary conditions for the walls, with the turbine’s operational state determined based on the direction of heat transfer. Additionally, mesh division techniques and turbulence models are selected based on the geometric dimensions and operating conditions of the compressor and turbine. The simulation results reveal that thermal inertia induces a shift in the dynamic characteristics of the rotor components. Under the same heat transfer conditions, variations in rotational speed have a minimal impact on the shift in the characteristic curve. The working fluid temperature inside the compressor components is lower, with a smaller temperature difference from the wall, resulting in less intense heat transfer compared to the turbine components. Overall, heat transfer accounts for only about 0.1% of the total enthalpy at the inlet. When heat exchange occurs between the working fluid and the walls, around 6–15% of the exchanged heat is converted into changes in technical work, with this percentage increasing as the temperature difference rises. Full article

Deep-sea hydraulic systems, powering a wide range of numerous deep-sea operating equipment, employ many poppet valves to adjust the pressure and flow rate, thereby realizing precise movements of the actuators. With greater depths and ambient pressures, the hydraulic oil viscosity increases exponentially, leading to a significant difference in the performance of the poppet valve compared to on-land usage and across varying depths. Based on the shear stress transport (SST) k-$\omega$ turbulence model and the dynamic mesh method, a computational fluid dynamics (CFD) model of the poppet valve was established. With the viscosity–pressure characteristics considered, the performance of the poppet valve was analyzed under different depths, different inlet flow rates, and different cracking pressures. The results indicate significant performance deterioration in poppet valves at increased depths, characterized by increased pressure loss and extended response rise time. At 11 km underwater, the pressure loss can be 7 MPa larger than the preset cracking pressure of 10 MPa, and the rise time is doubled compared with the land condition. It is recommended to use hydraulic oils with a lower initial viscosity and a slower increase in viscosity with pressure in deep sea conditions. Full article

To explore the influence of guide vanes on the energy loss of hydraulic turbines, a pump characterized by a simple structure and convenient operation was selected as the research subject. Entropy generation theory was utilized to analyze entropy generation losses at different flow rates, with a particular emphasis on the mechanisms in the impeller and draft tube. The findings indicate that turbulent entropy production dominates energy dissipation. Under the best efficiency point (BEP), the total entropy generation loss of Z₀ = 11 turbine was 7.18% and 5.76% lower than that of Z₀ = 7 and Z₀ = 9, respectively. The proportion of entropy generation loss in the impeller was highest under low-flow and optimal operating conditions, while the proportion of entropy generation loss in the draft tube was highest under high-flow conditions. In guide-vane-free turbines, the impeller’s high turbulent entropy generation rate was attributed to vortices and backflow caused by significant velocity gradients. For guide-vane-equipped turbines, high turbulent entropy generation rates arose from rotor–stator interactions and flow separation at blade inlets. Under high-flow-rate conditions, the entropy generation loss in the draft tube was significantly larger than that in other flow components, primarily due to vortices generated by excessive velocity circulation at the impeller outlet near the upstream draft tube flow passages, leading to high turbulent entropy generation rates. Full article

The electric submersible pump (ESP) is a critical component in subsurface resource extraction systems, yet the presence of gas in the working medium significantly affects its performance. To investigate the impact of impeller perforation on gas–liquid mixing and internal flow characteristics, unsteady numerical simulations were conducted based on the Euler–Euler multiphase flow model. The transient evolution of the gas phase distribution, flow behavior, and liquid phase turbulent entropy generation rate was analyzed under an inlet gas volume fraction of 5%. Results show that under part-load flow conditions, impeller perforation reduces the amplitude of dominant frequency fluctuations and enhances periodicity, thereby mitigating low-frequency disturbances. Under design flow conditions, it leads to stronger dominant frequencies and intensified low-frequency fluctuations. Gas phase distribution varies little under low and design flow rates, while at high flow rates, gas accumulations shift from the midsection to the outlet with rotor rotation. As the flow rate increases, liquid velocity rises, and flow streamlines become more uniform within the channels. Regions of high entropy generation coincide with high gas concentration zones: they are primarily located near the impeller inlet and suction side under low flow, concentrated at the inlet and mid-passage under design flow, and significantly reduced and shifted toward the impeller outlet under high flow conditions. The above results indicate that the perforation design of ESP impellers should be optimized according to operating conditions to improve gas dispersion paths and flow channel geometry. Under off-design conditions, perforations can enhance operational stability and transport performance, while under design conditions, the location and size of the perforations must be precisely controlled to balance efficiency and vibration suppression. Full article

This study addresses inlet flow distribution and pressure pulsation-induced vibration in LNG dual-pump parallel systems. We investigate an LNG dual-submerged pump tower system. Our approach combines computational fluid dynamics with vortex dynamics theory. We examine inlet flow characteristics under different flow conditions. Pressure pulsation propagation patterns are analyzed. System stability mechanisms are investigated. A 3D model incorporates inducers, impellers, guide vanes, outlet sections, and base structures. The SST k-ω turbulence model and Q-criterion vortex identification reveal key features. Results show minimal head differences during parallel operation. The inlet flow field remains uniform without significant vortices. However, local low-velocity zones beneath the base may cause flow separation at low flows. Pressure pulsations are governed by guide vane rotor–stator interactions. These disturbances propagate backward to impellers and inducers. Outlet sections show asymmetric pressure fluctuations. This asymmetry results from spatial positioning differences. Complex base geometries generate low-intensity vortices. Vortex intensity stabilizes at higher flows. These findings provide theoretical foundations for vibration suppression. Full article

The operational reliability of industrial cooling systems is critically compromised by the crystallization of ammonium chloride (NH₄Cl) in the terminal sections of heat exchangers and at air-cooler inlets. This study systematically investigated the deposition characteristics of NH₄Cl particles in hydrogenation air coolers, along with the factors influencing this process, using a combination of experimental analyses and CFD-DEM coupled simulations. Numerical simulations indicated that gas velocity is the primary factor that governs the NH₄Cl deposition behavior, whereas the NH₄Cl particle size significantly affects the deposition propensity. Under turbulent conditions, larger particles (>300 μm) exhibit a greater deposition tendency due to increased inertial effects. A power-law equation (R² > 0.75) fitted to the experimental data effectively predicts the variations in the deposition rates across tube bundles. This study offers a theoretical foundation and predictive framework for optimizing anti-clogging design and maintenance strategies in industrial air coolers. Full article

Outboard Dynamic-inlet Waterjets (ODW) are axisymmetric units, powered by a self-contained pump, that, by processing a uniform undisturbed streamtube, can operate more efficiently than conventional marine propulsors. This feature also provides methodological convenience, enabling accurate numerical investigations of the system alone using 2D axisymmetric models. Leveraging this property, the present study bridges the gap on the design principles required to tailor ODW geometries across multiple operating conditions. Reynolds-Averaged Navier Stokes (RANS) equations are solved, including turbulence and cavitation models, to draw the propulsor’s characteristic maps and identify two relevant operating points, set by the combination of a specified pump rotational regime with an advancing velocity. Simulations for these in- and off-design conditions are systematically performed over a database of 512 randomly sampled geometric variants. The corresponding results show that optimised shapes improving the inlet Pressure Recovery (PR) and nacelle drag at cruise conditions result in beneficial outcomes also at take-off operations, where lip cavitation may occur. Thus, analysing together the off-design PR and the cruise net force underscores their conflicting behaviour. In fact, while nacelles shortened by $12\%$ can reduce overall drag and enhance nominal net thrust by $2\%$, designs featuring a $34\%$ wider capture area improve off-design PR by over $1.5\%$, albeit at the cost of compromised propulsive efficiency under any operating range. Full article

In this study, the influence of window opening sizes and positions on wind-induced cross ventilation performance in an isolated gable roof building was numerically investigated using the k-ω SST turbulence model. The results obtained from numerical analyses to evaluate the ventilation efficiency of different configurations show that larger inlet openings significantly increase the ventilation rates and the WO5 model reaches the highest ventilation rate of 0.004089 m³/s with an improvement of 37.27% compared to the reference model. As with the WO1 model, smaller inlet openings limited the air intake, reducing ventilation efficiency and indoor air quality. In terms of outlet window opening sizes, the LO5 model showed the highest ventilation efficiency, improving ventilation by 28% compared to reference model, while smaller outlet openings, as in the LO1 model, were associated with significantly lower performance. Additionally, when evaluating window opening locations, configurations with higher exit openings generally exhibited superior ventilation rates. The best overall ventilation performance was achieved in the Upper-Lower configuration at 0.003129 m³/s. The findings emphasized the critical role of window design in natural ventilation performance. Larger and strategically located window openings optimize airflow, increase ventilation efficiency and improve indoor air quality, providing valuable information for energy-efficient building design. Full article

Static mixers have been widely used in marine research fields, such as marine control systems, ballast water treatment systems, and seawater desalination, due to their high efficiency, low energy consumption, and broad applicability. However, the turbulent mixing process and fluid–wall interactions involving complex structures make the mixing transport characteristics of static mixers complex and nonlinear, which affect the mixing efficiency and stability of the fluid control device. Here, the modeling and design optimization of the two-phase flow mixing and transport dynamics of a static mixer face many challenges. This paper proposes a modeling and problem-solving method for the two-phase flow transport dynamics of static mixers, based on the lattice Boltzmann method (LBM) and large eddy simulation (LES). The characteristics of the two-phase flow mixing dynamics and design optimization strategies for complex component structures are analyzed. First, a two-phase flow transport dynamics model for static mixers is set up, based on the LBM and a multiple-relaxation-time wall-adapting local eddy (MRT-WALE) vortex viscosity coupling model. Using octree lattice block refinement technology, the interaction mechanism between the fluid and the wall during the mixing process is explored. Then, the design optimization strategies for the flow field are analyzed under different flow rates and mixing element configurations to improve the mixing efficiency and stability. The research results indicate that the proposed modeling and problem-solving methods can reveal the dynamic evolution process of mixed-flow fields. Blade components are the main driving force behind the increased turbulent kinetic energy and induced vortex formation, enhancing the macroscopic mixing effect. Moreover, variations in the flow velocity and blade angles are important factors affecting the system pressure drop. If the inlet velocity is 3 m/s and the blade angle is 90°, the static mixer exhibits optimized overall performance. The quantitative analysis shows that increasing the blade angle from 80° to 100° reduces the pressure drop by approximately 44%, while raising the inlet velocity from 3 m/s to 15 m/s lowers the outlet COV value by about 70%, indicating enhanced mixing uniformity. These findings confirm that an inlet velocity of 3 m/s combined with a 90° blade angle provides an optimal trade-off between mixing performance and energy efficiency. Full article

In this study, the effect of inlet-turbulence intensity on the friction coefficient for the transitional boundary layer has been investigated computationally. For this purpose, two equation turbulence models of Std. k-ε, RNG k-ε, Std. k-ω, and SST k-ω have been compared with the Gamma–Theta (GT) transitional model, and it has been found that the Gamma–Theta model is the most consistent model with the experimental values of the ERCOFTAC T3A test case. Then, the effect of inlet-turbulence intensity on the friction coefficient has been computed by using this Gamma–Theta model. The transition from laminar to turbulence is shortened with increasing turbulence intensity by changing it from 1% to 10%. The most suitable inlet-turbulence intensity value with the experimental results of the ERCOFTAC T3A test case is found as Tu = 3.3%. Full article

Under extremely low-head conditions, the performance and stability of pump-turbine units are strongly influenced by the flow distortion caused by variations in guide vane opening. In this study, a pump-turbine model—representative of a domestic pumped storage power station—was investigated through a combination of experimental observations and three-dimensional unsteady numerical simulations employing the SST k-ω turbulence model. The analysis focused on characterizing the variations in turbulence kinetic energy, pressure pulsations, and impeller force fluctuations as the guide vane opening was altered. The results reveal that, with increasing guide vane opening, the turbulence kinetic energy within the impeller region is notably reduced. This reduction is primarily attributed to a decrease in energy losses along the suction surfaces of the blades and within the straight pipe section of the tailwater tunnel. Simultaneously, pressure pulsations were detected at multiple locations including the volute inlet, the blade-free zone, downstream of the conical pipe, and along the inner surface of the shaft tube. While most regions experienced a decline in pressure pulsation intensity with larger openings, the bladeless zone exhibited a significant increase. Moreover, force analysis at four distinct guide vane settings indicated that an opening of 41 mm resulted in relatively uniform fluctuations in the impeller forces. This uniformity suggests that an optimal guide vane configuration exists, which minimizes uneven stress distributions and enhances the operational stability of the pump-turbine under extremely low-head conditions. These findings offer valuable insights for the design and operational optimization of pump-turbine systems in pumped storage power stations. Full article

During crude oil exploration and extraction, the presence of H₂S not only poses a threat to operational safety but also accelerates equipment corrosion, highlighting the urgent need for efficient and cost-effective processing solutions. This study employs a coupled numerical simulation approach that integrates computational fluid dynamics (CFD) and population balance models (PBM) to systematically investigate the multiphase flow characteristics within SK static mixers. By embedding mass transfer rates and reaction kinetics equations for hydrogen sulfide and sodium hypochlorite into the Euler-Euler multiphase flow model using user-defined functions (UDFs), the effects of equipment structure on the efficiency of the crude oil desulfurization process are examined. The results indicate that the optimized SK static mixer (with 15 elements, an aspect ratio of 1, and a twist angle of 90°) achieves an H₂S removal efficiency of 72.02%, which is 18.84 times greater than that of conventional empty tube reactors. Additionally, the micro-mixing time is reduced to 0.001 s, and the coefficient of variation (CoV) decreases to 0.21, while maintaining acceptable pressure drop levels. Using the CFD-PBM model, the dispersion behavior of droplets within the static mixer is investigated. The results show that the diameter of the inlet pipe significantly affects droplet dispersion; smaller diameters (0.1 and 1 mm) enhance droplet breakup through increased shear force and turbulence effects. The findings of this study provide theoretical support for optimizing crude oil desulfurization processes and are of significant importance for enhancing the economic efficiency and safety of crude oil extraction operations. Full article

The present study investigates the flow dynamics surrounding offshore wind turbine OWT foundations, focusing on the interaction of wind and water flows with two prevalent foundation types: mono-pile and tripod designs. Computational simulations and analyses were conducted on the substructures of these OWTs using the ANSYS-Fluent v16.5 software package. The primary objective was to predict critical parameters, including directional drag force coefficients, interface velocities, and pressure distributions. To model realistic oceanic conditions, pseudo-periodic wave patterns were implemented at the inlet boundary. The flow regime was characterized by logarithmic vertical velocity profiles at low interfacial velocities, ranging from 2.23 m/s to 3.01 m/s. This computational approach revealed anisotropic constraints imposed on the foundations under unidirectional flow conditions. The drag coefficients obtained from the simulations highlighted significant vertical flux exchanges in proximity to the OWT structures, with a particularly pronounced downward flow near the tripod foundation design. Additionally, the study demonstrated that variations in wind speed within the specified range did not substantially impact pressure distributions or strain rates. However, these changes were found to influence skin friction coefficients, indicating a sensitivity of these hydrodynamic parameters to wind speed variations. The analysis of flow streamlines around the mono-pile foundation showed a smooth and well-defined pattern, whereas the flow around the tripod foundation exhibited more complex, interleaved, and turbulent streamlines. This distinction in flow behavior is believed to contribute to the observed downward vertical flux exchanges near the tripod. Full article

Diaphragm valves play a crucial role in controlling fluid flow in piping systems, and their hydraulic performance directly impacts system efficiency. This study employs numerical simulations using OpenFOAM v8 to investigate the hydraulic characteristics of a diaphragm valve, focusing on the effects of inlet boundary conditions and turbulence models on head loss. At the maximum valve opening, two inlet conditions of OpenFOAM, flowRateInletVelocity and timeVaryingMappedFixedValue, were compared. Results show that the flowRateInletVelocity inlet condition yields simulation results in excellent agreement with experimental data, validating its reliability. Five turbulence models (Standard k-ε, Realizable k-ε, RNG k-ε, SST k-ω, and Spalart-Allmaras) were evaluated, revealing that the SST k-ω model offers the highest computational accuracy in capturing flow field details and head loss, while the Spalart-Allmaras model demonstrates significant discrepancies. Further analysis under varying valve openings and flow rates identifies an exponential relationship between head loss and value opening, with the most pronounced changes occurring below 50% opening. These findings provide a theoretical basis for optimizing diaphragm valve designs and enhancing the accuracy of CFD simulations in hydraulic engineering applications. Full article

Theoretical and experimental investigations were conducted to predict permeate flux in direct contact membrane distillation (DCMD) modules equipped with turbulence promoters. These DCMD modules operate at moderate temperatures (45 °C to 60 °C) using a hot saline feed stream while maintaining a constant temperature for the cold inlet stream. The temperature difference between the two streams creates a gradient across the membrane surfaces, leading to thermal energy dissipation due to temperature polarization effects. To address this challenge, 3D-printed turbulence promoters were incorporated into the DCMD modules. Acting as eddy promoters, these structures aim to reduce the temperature polarization effect, thereby enhancing permeate flux and improving pure water productivity. Various designs of promoter-filled channels—with differing array configurations and geometric shapes—were implemented to optimize flow characteristics and further mitigate polarization effects. Theoretical predictions were validated against experimental results across a range of process parameters, including inlet temperatures, volumetric flow rates, hydraulic diameters, and flow configurations, with deviations within 10%. The DCMD module with the inserted 3D-printed turbulence promoters in the flow channel could provide a relative permeate flux enhancement up to 91.73% under the descending diamond-type module in comparison with the module of using the no-promoter-filled channel. The modeling equations demonstrated technical feasibility, particularly with the use of both descending and ascending hydraulic diameters of 3D-printed turbulence promoters inserted into the saline feed stream, as compared to a module using an empty channel. Full article